1. Field of the Invention
The present invention relates to a coordinate detecting apparatus, and more particularly, to an analog type apparatus using a resistive film having a plane on which the position of a point whose coordinate is to be detected is indicated by applying a load impedance to the position.
With the development of office automation, there is a growing need for simple means for inputting the coordinate data of a point into a computer system. A soft-key, which is a coordinate data inputting means comprising an input panel stacked on the surface of a display device, permits the input of the coordinate of a point on the panel by only applying the touch of a fingertip, etc. to the panel. Thus, the soft-key can greatly facilitate the man-machine interaction in a computer system. For example, the selection of a menu on a display device or inputting of a pattern to a computer system can be performed without using a keyboard and only touching the input panel or writing the pattern on the panel. An exemplary application of such a soft-key has been realized in a window machine for banks or a seat reservation terminal for travel agencies. The use of the soft-key is expected to increase more and more in view of the trend toward integrated digital information networks where easy-to-operate terminals for nonspecialized persons are essential.
2. Description of the Prior Art
The coordinate data inputting technology for the above-described soft-key, is a digital type apparatus comprising a plurality of sensors disposed to form a matrix arrangement on a plane. By indicating one or a plurality of the sensors with a touch of a fingertip or pen, the coordinate data corresponding to the position(s) of the sensor(s) is applied to a control unit. However, detection accuracy in the coordinate data obtained by using such digital type means depends on the number of the sensors per inch, and is insufficient for applications requiring high resolution necessary to the inputting of a fine or complicated pattern.
Another type of coordinate inputting means using an input panel having a resistive film was first disclosed in Proc. 1971 SID, under the title of "Conducting Glass Touch Entry System" by R. K. Marson, followed by various modifications, for example, disclosed in Proc. 1973 SID, under the title of "The Analog Touch Panel" by J. A. Turner et al. In this method, a resistive film constituting an input panel is supplied with current from both ends, and the point whose coordinate on the input panel is to be detected is indicated by applying a load impedance to the resistive film at the point. The coordinate of the point is given as a function of the ratio of respective currents flowing from both ends to the load. Thus, the coordinate is originally acquired as analog data. Therefore, higher detection accuracy can be attained when compared with the above-mentioned digital type, and the detection accuracy primarily depends on the quantization characteristics of the A-D (analog-to-digital) converter used in the conversion of the original analog data to digital data.
The principle of an analog type coordinate detecting method as above will be described with reference to FIG. 1.
FIG. 1 is a circuit block diagram of a conventional coordinate detecting apparatus of the aforesaid analog type. Referring to FIG. 1, the output terminal 5 of a voltage source 8 is connected to one end 2 of a uniform resistive film 1 via a current measuring means 9. The output terminal 5 of the voltage source 8 is also connected to the other end 3 of the resistive film 1 via another current measuring means 10. Another output terminal of the voltage source 8 is connected to the ground 7. The output of the current measuring means 9 is connected to an analog-to-digital converter (ADC) 11, while the output of the current measuring means 10 is connected to another ADC 12. The outputs of both ADCs 11 and 12 are individually connected to a control unit 13.
When a load 6 which is a capacitor, for example, and has a definite impedance with respect to the ground 7, is applied to the position of a point 4 on the resistive film 1, respective currents, which are supplied by the voltage source 8, flow into the load 6 via the ends 2 and 3.
Since the resistive film 1 has a uniform resistivity, the resistance between two arbitrary points on the resistive film 1 is proportional to the distance between the points. When assuming the coordinate of the end 2 to be 0 while the coordinate of the end 3 to be 1, the coordinate x of an arbitrary point 4 on the resistive film 1 is represented by the equation EQU x=R.sub.x /(R.sub.x +R.sub.1-x) (1)
where, 0.ltoreq.x.ltoreq.1, and, R.sub.x and R.sub.1-x denote respective resistances between the end 2 and point 4 and between the end 3 and point 4.
The voltage drop across the resistance R.sub.x is equal to that across the resistance R.sub.1-x, therefore, EQU R.sub.x .multidot.I.sub.x =R.sub.1-x .multidot.I.sub.1-x ( 2)
where, I.sub.x and I.sub.1-x denote respective currents flowing through the ends 2 and 3.
Hence, equation (1) can be represented as follows: EQU x=I.sub.1-x /(I.sub.x +I.sub.1-x) (3)
and thus, the coordinate of the point 4 can be determined by the currents I.sub.x and I.sub.1-x, both measured by using the current measuring means 9 and 10. In the apparatus shown in FIG. 1, the value of the currents I.sub.x and I.sub.1-x are converted to corresponding digital data by the respective ADCs 11 and 12, and processed by the control unit 13 to provide a digital coordinate data, according to equation (3).
The conventional coordinate detecting apparatus shown in FIG. 1 uses current measuring means 9 and 10, each of which usually comprises an operational amplifier serving to provide a voltage output signal corresponding to the measured current. The coordinate detecting apparatus also requires analog-to-digital converters (ADCs) 11 and 12 of high quantization accuracy. As a result, the apparatus inevitably has a complicated circuit configuration and high cost.